fix(ci): fix Go stdlib R2 upload workflow (GOROOT capture + build tag conflict)

[shivasurya]

Summary

• `$GOROOT` is not exported to the shell by `actions/setup-go` — only `go` is added to `PATH`
• All 9 "Save Go X.XX GOROOT" steps captured an empty string, causing every version to be skipped with `Warning: GOROOT_1_XX is not set`
• Fix: replace `$GOROOT` with `$(go env GOROOT)` which queries the active Go binary on PATH

Test plan

• Push trigger added on `fix/go-stdlib-r2-goroot-capture` branch — workflow runs on this PR branch directly without needing to merge
• Verify "Verify Go installations" step shows all `GOROOT_1_XX` populated (not ``)
• Verify registries upload to R2 at `assets.codepathfinder.dev/registries/go{version}/stdlib/v1/manifest.json`

🤖 Generated with Claude Code

[safedep]

SafeDep Report Summary

No dependency changes detected. Nothing to scan.

_{This report is generated by SafeDep Github App}

[github-actions]

Code Pathfinder Security Scan

No security issues detected.

Metric	Value
Files Scanned	3
Rules	38

---

_{Powered by Code Pathfinder}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.32%. Comparing base (b28db49) to head (248d6bd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #559   +/-   ##
=======================================
  Coverage   83.32%   83.32%           
=======================================
  Files         143      143           
  Lines       17782    17782           
=======================================
  Hits        14817    14817           
  Misses       2426     2426           
  Partials      539      539           

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